The present invention relates to a method and apparatus for forming glass preforms from core glass and cladding glass compositions having narrow working ranges and little resistance to devitrification.
Certain glasses such as halides, chalcogenides, oxy-halides, lead silicates, phosphates, borates and the like, which exhibit narrow working ranges, are difficult to form into fibers. By "narrow working range" is meant that a change in temperature of only a few degrees Centigrade can change the viscosity of a molten glass to such an extent that the glass freezes, i.e. it changes from the liquid to the solid state. Many of these glasses also exhibit low melting point temperatures, and they often readily devitrify, thereby requiring short residence time at elevated temperature to prevent devitrification. Glasses having narrow working ranges are exemplified by those taught in U.S. Pat. Nos. 4,314,031, 4,142,986, 4,405,724, 4,537,864, 4,668,641 and 4,752,593.
Such glasses have generated considerable interest in optical applications such as fibers, lasers and the like. Fluoride glasses, for example, are attractive candidates for transmission optical fibers, because of their low intrinsic scattering loss properties; fluoride glasses can also function as host materials for lasing dopants. However, it has been difficult to form optical fibers, fiber lasers and the like from such glasses. Conventional manufacturing techniques, such as chemical vapor deposition, cannot presently be used to make fluoride glass preforms since suitable precursor compounds have not yet been identified.
Moreover, transmission loss in such glasses can be inordinately high because of thermal compositional fluctuations, phase separation, scattering sites such as crystallites, as well as geometric variations in the size of the fiber core. Conventional techniques for manufacturing fluoride glass preforms produce fibers having these defects.
In accordance with one fabrication method, the fluoride cladding glass melt is poured into a cylindrical container which is rotated to uniformly distribute the cladding glass about the inner surface of the container wall. After the cladding glass has solidified, a rod-shaped region of core glass can be disposed within the cladding glass by pouring a melt of core glass into the cladding glass tube. The inner portion of the cladding glass tube is reheated by the incoming core glass melt. Large crystals can be formed by this reheating of the cladding glass, and these cause scattering in the resultant fiber.
In another technique, a rod-shaped region of core glass and a cladding glass tube are separately cast. The core rod is thereafter placed within the cladding glass tube to form a rod-in-tube preform wherein crystals have not been formed due to reheating the glass. However, fibers drawn from rod-in-tube preforms contain defects at the core-clad interface, which can greatly increase attenuation.
A double crucible method of forming an optical fiber preform is disclosed in U.S. Pat. No. 4,729,777. Inner and outer crucibles are charged with core and cladding glass. After initially being heated to a relatively high temperature, the melts are cooled to a temperature at which they become viscous enough for drawing. When drawing narrow working range glasses by the double crucible method, control of glass flow has been a problem because of the sensitivity of viscosity to temperature. The glass that flows immediately adjacent the core and cladding orifices has a sufficiently long residence time to permit crystallization to begin at the outer surface and at the core/clad interface of the preform. The crystals can then grow later when the preform is drawn into a fiber. That patent states that the method will not cause crystallization of the glass so long as it has a composition of relatively high stability.
In accordance with the teachings of U.S. Pat. No. 4,925,475 a small diameter vessel is located in the top portion of a large diameter vessel. After the vessels are heated to a temperature above 800.degree. C., core glass and cladding glass is loaded into the vessels. After the glasses have been melted, gas begins to cool the outer vessel at a rate such that the inner portion of the cladding glass melt is in the molten state when its outer portion solidifies. When the thickness of the cooled and solidified portion of the cladding glass reaches the desired value, a plug is pulled from the bottom of each vessel. The axial cladding glass region, which is still liquid, runs out, and the core glass melt is simultaneously introduced into the remaining void. This method is disadvantageous in that the solidified cladding glass is reheated by the hot incoming core glass melt, thereby increasing the chance of subsequent crystal growth. Another possible disadvantage of this method is that during the time it takes to drain and refill the core region, additional cooling can occur; this could result in a variation in the core dimensions along the long axis of the preform.